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Flight Tests Mark Key Stage For S-64 Composite Blade Program

Flight tests of Erickson Inc.’s long-running advanced composite main rotor blade upgrade for the Sikorsky S-65/CH-54 heavy-lift helicopter have accelerated, with completion now targeted by late summer and certification anticipated about six months later.

The new blade is designed to significantly improve aircraft performance, particularly fuel burn and payload capability at altitude. It is also aimed at development of a single main rotor blade that can be installed on four different derivatives; the CH-54A/B and the S-64E/F.

Flight tests show double-digit efficiency improvement and higher resistance to resonance and shudder

However, testing and development of the blade has taken longer than expected. Started by Erickson and Helicopter Transport Services in 2010, the program was originally slated for certification by the end of 2016. But delays cropped up after delamination occurred in some of the initial flight-test blades and, following a set of other unspecified hurdles, some key tests were pushed back into early 2018.

“There was a series of events, malfunctions and problems with some of the blades coming out of manufacturing,” says Erickson chief pilot Randy Erwin. “We finished up strain-gauge certification for the composite blades on the S-64F model, but missed the window [for high-altitude tests] in Leadville, Colorado, and made the decision to stop and do the S-64E model.” Although delamination was noted on three blades within a few hours of starting flight tests, “the engineers worked on it and found [this occurred] only in particular blades in which the manufacturing had been different to the others,” he adds.

Blade development was carried out by Newport News, Virginia-based Advanced Technologies Inc., with some blades for flight testing produced by Composite Technology Inc., a Texas-based Sikorsky company now owned by Lockheed Martin, and others produced at a newly established composite manufacturing facility set up by Erickson in Medford, Oregon. All production issues have now been resolved, and vibration and strain gauge testing has been completed on the baseline metallic and composite blades on both the S-64E and F helicopters.

“Now it is time for performance testing,” says Erwin. The instrumented S-64F test aircraft, which shares the same rotor head as the former CH-54B military variant, is in South America where, following low-level tests in Tacna, Peru, and high-altitude work in Cusco, Peru, it is scheduled to appear at the upcoming FIDAE airshow in Santiago, Chile. The S-64E test aircraft, which has a common rotor head with the CH-54A, is meanwhile due to conduct high-altitude tests at Leadville in June following lower altitude work in Medford.

The new blades feature advanced airfoil technology, twist and swept tips for improved performance and extended life. Although testing is ongoing, initial performance results are encouraging, says Erwin. Despite being 8 lb. heavier per blade than the original hollow extruded aluminum rotor because of weights added for rigidity, he says the aerodynamic benefit results in a 13% improvement in efficiency in the S-64F.

“I was really shocked,” says Erwin. During tests of the S-64F at maximum gross weight, the helicopter was brought to an initial hover at 200 ft. at a weight of a little more than 48,000 lb—about 1,000 lb. higher than the standard gross weight—in order to reach the appropriate test condition. Adjusting for the 1,500-ft. altitude of the helipad and the 20C (68F) temperature at the time of the test, the helicopter was hovering at an equivalent density altitude of almost 3,000 ft.

“The performance charts said that we were torque-limited, meaning the horsepower the transmission would accept was far below what the engines would produce,” he says. “The charts would tell you at 3,000-ft. density altitude with a max power of 7,900 hp, you should be at 46,000 lb. So, with metal blades we would have been torque-limited at 82.3%, and instead we were hovering at 68% with 48,000 lb. In Peru we are drawing new performance limit charts.”

Aside from greater lift, the composite blades have proven to be “benign” to ground resonance, a phenomenon to which helicopters with articulated rotor systems are particularly susceptible. The blade also negates most of the “shudder,” excessive vibration that can affect large helicopters during periods of translational lift as they move from hovering to forward flight and vice versa. The shudder is caused by the tail rotor passing through vortices shed from the main rotor. “It beats the hell out of the airframe, and the heavier you are the more pronounced it is. In the [S-64] Skycrane it is so bad that, if the pilot is not adept at staying out of it, the instrument panel vibrates so much it can be hard to read. These blades have almost completely eliminated that shudder,” says Erwin.

From a maintenance and operational tempo perspective, the composite blades are also beneficial, he adds. The spars inside the hollow metal blades are sealed to allow nitrogen to be stored inside at 10 psi. “As the blade[s] get older, over time they will fatigue and maybe crack. There is a little pressure sensor in the blade root, so if the spar vents that pressure you will see it,” he says. In addition, flights of 5 hr. or more are not permitted without checking the spar pressures. “With a composite blade you don’t need that,” Erwin notes.

Fuel burn is also expected to be reduced by the new blades, though the company, which previously forecast a 3-5% reduction, is not specifying the figures until flight tests are complete. “We are currently burning 525 gal./hr., so we are going to see a big improvement in that,” he says.

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